In this April 10, 2015 photo, Choi Yun-ah, sister of Danwon High School student Choi Yun-min, who died in the sinking of ferry Sewol, hangs a note of tribute for missing passengers and victims on a tree in Ansan, South Korea. Siblings, she says, often have no one else to turn to because they are afraid of putting more of a burden on their grieving parents. “Siblings are victims too, but no one in the world knows about it.” The Sewol ferry sank on April 16, 2014, killing 304 people including 250 Danwon High School students on a school trip. (AP Photo/Ahn Young-joon)
(The Associated Press)

In this April 10, 2015 photo, Choi Yun-ah, sister of Danwon High School student Choi Yun-min, who died in the sinking of ferry Sewol, is interviewed by the Associated Press in Ansan, South Korea. Siblings, she says, often have no one else to turn to because they are afraid of putting more of a burden on their grieving parents. “Siblings are victims too, but no one in the world knows about it.” The Sewol ferry sank on April 16, 2014, killing 304 people including 250 Danwon High School students on a school trip. (AP Photo/Ahn Young-joon)
(The Associated Press)

In this April 12, 2015, photo, survivor Yang Jeong-won, a student who was rescued from the sunken Sewol ferry, puts her head on her desk inside a classroom at Danwon High School in Ansan, South Korea, which has become a memorial for her classmates who were killed. "Field Trip" is still written in big letters on a calendar hanging on Yang’s old classroom wall. The letters cover four days in April last year and mark one of the highlights of the year - a trip, by ferry, to a southern resort island. The sight still manages to shock Yang, one of the few students to survive a disaster that continues to horrify South Korea. The ferry disaster on April 16, 2014, killed 304 people including 250 Danwon High School students on a school trip. (AP Photo/Lee Jin-man)
(The Associated Press)

ANSAN, South Korea – "Field Trip" is still written in big letters on a calendar hanging on Yang Jeong-won's old classroom wall.

The letters cover four days in April last year and mark one of the highlights of the year — a trip, by ferry, to a southern resort island. The sight still manages to shock Yang, one of the few students to survive a disaster that continues to horrify South Korea.

For many of the second-year high school students, the trip marked a last bit of freedom before nearly two years of grueling preparation for college entrance exams. Teachers and other students waved at them from school windows as 17-year-old Yang and her classmates boarded buses at Danwon High School in the city of Ansan, about an hour's drive south of Seoul.

Girls sang along to loud music and took selfies as they drove to the port of Incheon. Once heavy fog cleared, the Sewol, a 6,852-ton ferry, set off with 476 people on board, including 325 students and 14 teachers from Danwon.

That night, Yang and her classmates threw a surprise birthday party for a beloved teacher, Kim Cho-won, who turned 26 on the ship. A few minutes before midnight, the students lured Kim to a cabin and greeted her with a birthday cake and a happy birthday song and a picture.

Several hours later, on the morning of April 16, 2014, the ferry listed, flipped upside down and sank. More than 300 people died, many of them students trapped in cabins because the crew ordered them to stay put, even as the captain and others jumped on to an early coast guard boat. As the ship listed, Yang smashed into a wall. Students screamed and called their parents, cabinets falling onto them.

Yang, who had on a life jacket, managed to get to an exit and called out to rescuers, who saved her.

Out of 35 people in the birthday photo, she was one of only eight who lived. The teacher, Kim, didn't make it.

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A year later, Yang still can't believe what happened to her friends. She's now in her final year of high school, studying design and animation, part of a senior class that shrank from 338 to just 88 students.

"I sometimes wonder if some of the other students are still in the classroom, but no, this is all we have now," she said.

Yang, a freckled, short-haired girl, takes medicine for anxiety. The feeling of the tilting ship still traumatizes her, and when she was on the 11th floor of a hospital last year, she felt like the building was leaning to one side. She grabbed her younger brother's hand and ran down the steps to the first floor.

"If I had known this would happen I would have tried to be better friends with more kids," she said during a two-hour interview with The Associated Press.

She feels guilty about surviving.

"I came out of this alive, but my friends didn't," she said. "I once dreamed that relatives of those who died came to kill the survivors."

Jang Dong-won, the father of another student who was rescued, said that surviving students are still traumatized by memories of their friends during the last minutes of the sinking ship.

"One child missed a friend's hands as the person was swept away by waves," Jang said. "That last glance by the student who couldn't move as a vending machine fell on them."

They also are made to feel guilty by those around them.

"If they smile even just a little bit, they hear, 'How can you smile'" when most of their friends were killed, he said.

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The ferry sinking also has dramatically changed the lives of siblings of victims.

After Choi Yun-ah, 24, lost her 17-year-old sister, Yun-min, in the sinking, she quit her office job. She then reached out to other siblings of ferry victims from Danwon. Siblings, she says, often have no one else to turn to because they are afraid of putting more of a burden on their grieving parents.

"Siblings are victims too, but no one in the world knows about it," Choi said. She has made art out of messages from 51 siblings to their dead sisters and brothers.

She has also overcome her previous reticence and become more vocal about the sinking.

"Kids died because they were asked to stay still," Choi said. "If I stay obedient and listen to what grown-ups tell me to do, it will damage me, so I now speak my mind."

After Park Bona's 17-year-old brother died in the ferry, Park stopped caring about the things most other 20-something South Koreans are obsessed with.

"After I lost Seong-ho, working at a big company, graduating from college, making a lot of money — it all became meaningless to me," Park said as she sat in a makeshift church near a memorial altar for the victims. "What does it even mean to get married in a country that can't even protect my brother?"

Park, a 21-year-old who was majoring in Korean language and literature in college, is now preparing to transfer to another university. She wants to help society remember the dead teenagers and to make the country a safer place.

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Yang, meanwhile, is still wrestling with why friends who could have been saved had to die.

She feels angry at the ferry operator. The ship became top-heavy after a redesign added more cargo space. She blames the government for not regulating ferry operators more carefully.

Yang is also enraged by the ship's crew, who escaped when students were trapped because they were told to wait inside.

"They should be punished as heavily as possible," she said. "It would be very hard for me to forgive them."